Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Within one year, Amortized Cost	$ 62,627	$ 69,177
After one year through five years, Amortized Cost	89,766	80,593
After five years through ten years, Amortized Cost	67,212	53,835
After ten years, Amortized Cost	157,386	165,406
Available-for-sale, Amortized Cost	391,687	387,665
Within one year, Fair Value	63,083	69,938
After one year through five years, Fair Value	94,312	85,829
After five years through ten years, Fair Value	69,638	57,829
After ten years, Fair Value	169,843	181,631
Available-for-sale, Fair Value	411,904	414,279
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Without single maturity date, Amortized Cost	14,696	18,654
Available-for-sale, Amortized Cost	14,696	18,654
Without single maturity date, Fair value	15,028	19,052
Available-for-sale, Fair Value	$ 15,028	$ 19,052